What is meant by the line of action of force?

A horizontal force $‘F’$is applied to keep the block stationary on a frictionless inclined plane. Find the angle of incline, for which the applied force is equal to the weight of body: 

$1 \mathrm{kg} \mathrm{f} =$ _____ $\mathrm{N}$

(Choose from: $98 / 9.8 / 49 / 4.9$)

A person is holding a briefcase. Draw the force acting on the briefcase when the briefcase is at rest.

Discuss the effects a force can produce.

Define a contact force.

$\mathrm{kg}-\mathrm{wt}$ is a unit of force.

Discuss two effects a force can produce.

What is a resultant force?

It is difficult to balance our body when we accidentally slip on a peel of banana. Explain why? 

What causes acceleration?

Define force.